The invention relates to an automatic exposure controller for camera of TTL direct photometry type, and more particularly, to an automatic exposure controller for use in a camera of TTL direct photometry type in which light from an object being photographed which is reflected from both or either one of a shutter blind surface and a film surface is measured to determine a proper exposure period automatically.
A photometric device used in a camera of TTL photometry type determines light from an object being photographed which is incident on a film through a taking lens, for the purpose of providing a proper exposure. Ideally, the photometry should be performed by a photometric, photoelectric transducer element which is located on a film surface or on a point equivalent thereto, for example, on a shutter blind surface which is used in a focal plane shutter. This is infeasible in practice, and hence a technique is usually employed which determines light from an object being photographed which is reflected by a film surface or a shutter blind surface which is equivalent thereto, and this technique is commonly referred to as TTL direct photometry. FIG. 1 shows an exemplary arrangement of a camera of such photometry type. Below a movable mirror 1 and at a location out of a taking light path is a photometric or photoelectric transducer element 4 which is disposed so as to be opposite a first shutter blind 2 and a film 3. As the movable mirror 1 moves upward to a phantom line position 1A, the light passing through a taking lens 8, which previously been fed through a focussing glass 5, a pentaprism 6 and eyepiece 7 for observation by a photographer, is now directed to the first shutter blind 2, which reflects such light for photometry by the transducer element 4. Subsequently, as the first blind 2 begins to run to expose the film 3, the transducer element 4 also determines reflected light from the photosensitive surface of the film 3. An exposure control is achieved in response to a photometric output from the transducer element 4.
As is well recognized, where a focal plane shutter utilizes blinds, the film surface is initially covered by the first shutter blind which is formed by black cloth. As the first blind moves across an image field in response to a shutter release operation, the film surface which has been covered by the first blind becomes exposed. After a proper exposure period, a second shutter blind which is also formed by a black cloth begins to cover the exposed film surface again.
During a high speed shutter operation, the second blind begins running while the first blind is still running, thereby enabling a reduced exposure period to be obtained. It will be recognized that as the exposure period reduces, the width of a slit defined by the first and the second blind also reduces. When a proper exposure is to be determined by photometry of light from an object being photographed which light is reflected by both the first shutter blind of the blind shutter operating in the manner mentioned above and by the film surface, reflected light from the running first blind surface is initially measured, followed by the measurement of reflected light from the film surface which is being exposed. However, because of difference in the reflectivity of the blind surface and the film surface, the amount of light incident on the transducer element 4 changes significantly as shown in FIG. 2, as the first blind runs. Specifically, at time t.sub.1 which is near the beginning of the running of the first blind, the amount of light incident on transducer element 4 is at a low level l.sub.1, and increases as the blind continues to run until it reaches a higher level l.sub.2 at time t.sub.2 when the running of the first blind is completed. This is attributable to the fact that the blind surface is darker than the film surface, on the order of approximately three steps of EV-values. Thus it will be apparent that accurate exposure period cannot be calculated by directly integrating a photocurrent produced by the transducer element 4 in proportion to the amount of incident light.
In a conventional exposure control circuit, a technique is employed to assure a uniform photocurrent produced by the transducer element 4 independently of the position of running of the shutter blind, as by providing substantially equal reflectivity for the blind surface and for the film surface. However, to practice this technique, a material exhibiting the same reflectivity as the film surface is printed on the first blind surface in a given pattern. However, a shutter blind has its front surface formed by a cloth and its rear surface by a rubber-lined cloth, and this makes it very difficult to treat the surface as by printing in the manner mentioned above, resulting in a very expensive arrangement. In addition, the pattern being printed may change from product to product, causing a variation in the reflectivity. Furthermore, since a shutter blind is wound up at a high rate, the blind surface has a poor planarity, which may cause the patterned printing to be exfoliated, presenting a problem of maintenance.
Additionally, in order to reduce the incidence of stray light rays into the mirror box, leakage of light onto a film or to prevent a ghost or flare from occurring, cameras are usually internally provided with a black delustering printing. It will be seen that printing a reflecting pattern on the first blind surface interfers with the extinction effect of such delustering printing, giving rise to the occurrence of ghost and flare. Moreover, the reflectivity of the film surface changes slightly from film to film, and hence there occurs a varying difference in the reflectivity between the blind surface and the film surface as the film is changed. This produces an error, though small, in the exposure.
To provide an automatic exposure control which corrects for an error in the exposure resulting from the differential reflectivity of the film, a prior art includes an arrangement (see Japanese Laid-Open Patent Application No. 46,724/1978) including a first photometric circuit which is used for exposure control, and a second photometric circuit which is used to correct for a differential reflectivity of the film. There is provided light emitting element which directs its light to the film, the reflection from which is determined by the second photometric circuit to provide a photometric output, which is used to correct the magnitude of an exposure control output from the first photometric circuit. However, this arrangement requires a pair of photometric elements and a complex circuit arrangement, resulting in an increased cost.